A transmission that is designed for longitudinal mounting and is of countershaft structure usually has an input shaft, at least one countershaft and an output shaft. The input shaft can be connected to the driveshaft of the drive engine and separated therefrom by an engine clutch which acts as a starting and shifting clutch. The countershaft is arranged with its axis parallel to the input shaft and is in permanent driving connection therewith by way of an input constant usually formed by a spur gear pair with two fixed wheels arranged in a rotationally fixed manner on the respective transmission shaft (input shaft and countershaft). The output shaft is arranged axis-parallel to the countershaft and coaxially with the input shaft, and can be connected selectively to the countershaft by way of a number of gear steps with different transmission ratios. The gear steps are usually in the form of spur gear steps, each comprising a fixed wheel arranged in a rotationally fixed manner on one transmission shaft (countershaft or output shaft) and a loose wheel mounted to rotate on the other transmission shaft (output shaft or countershaft). To engage a gear, i.e. to form a driving connection between the countershaft and the output shaft with the transmission ratio of the spur gear step concerned, a gear clutch is associated with each loose wheel. The loose wheels of adjacent spur gear steps are usually arranged at least in pairs on the same transmission shaft, so that the gear clutches can correspondingly be combined in pairs in dual shifting elements, each having a common shifting sleeve.
The shifting sequence for an upshift from a gear under load to a higher, target gear generally begins when the torque delivered by the drive engine is reduced and approximately at the same time the engine clutch is opened, before the loaded gear is disengaged. This is followed by synchronization of the target gear, in which the input rotational speed, i.e. the speed determined by that of the input shaft or the countershaft at the input-side part of the gear clutch of the target gear, is reduced to the synchronous speed at the output-side part of the gear clutch of the target gear, which is determined by the rotational speed of the output shaft. Thereafter the target gear is engaged and then, at approximately the same time, the engine clutch is closed and the torque produced by the drive engine is increased again.
In automated transmissions the input rotational speed is usually detected by a speed sensor arranged on the input shaft, whereas the output speed is detected by a speed sensor arranged on the output shaft. For comparability of the two speeds it is necessary to relate them to a common transmission shaft, i.e. to convert them correspondingly. However, since particularly when the loose wheels on the countershaft and the output shaft are arranged in alternating pairs it would be relatively complicated to convert the rotational speeds in each case to the respectively relevant transmission shaft associated with the gear clutch of the target gear concerned, it is usual to relate the two speeds, in each case independently of the arrangement of the loose wheel concerned, uniformly to the same transmission shaft, preferably the input shaft. For this it is only necessary to convert the output rotational speed detected at the output shaft, by multiplication by the gear ratio of the target gear and the gear ratio of the input constant to the input shaft, whereas the input speed detected at the input shaft itself can be retained unchanged. Here the rotational speed conversion, which is known per se, will not be explained explicitly; rather, the input speed and the output speed will be understood to mean the respective rotational speeds already related to a common transmission shaft, in particular the input shaft.
In general, compared with gear clutches synchronized by means of friction rings and locking teeth, unsynchronized gear clutches known as claw clutches have a considerably more simple structure, lower production costs and more compact dimensions, and are substantially less prone to wear and defects. In an automated transmission fitted with claw clutches, during an upshift the target gear is preferably synchronized by means of a centrally arranged, controllable brake device, such as a transmission brake functionally connected to the input shaft or to the countershaft. Compared with control-path-dependent, adjustment-speed variable and adjustment-force-variable control of a shift-control element for synchronizing and engaging a synchronized target gear, the control of a transmission brake and of a shift-control element for synchronizing and engaging an unsynchronized target gear is comparatively simple since in essence the sensor data from the rotational speed sensors on the input and output shafts are sufficient for that purpose.
A typical transmission brake of an automated transmission of countershaft design is described, for example, in DE 10 2010 002 764 A1 with reference to FIG. 4 thereof. This known transmission brake is in the form of a pneumatically actuated disk brake and is arranged on the engine-side end of the countershaft of the transmission. The disks of the transmission brake are connected in alternation in a rotationally fixed manner, by means of inner and outer locking teeth, to the countershaft and to a brake housing mounted fixed on the transmission housing. The transmission brake is actuated by means of a piston arranged to move axially in a brake cylinder, which piston is acted upon axially on the outside by the controllable control pressure in the pressure chamber of the brake cylinder and is thereby pressed against the disks in opposition to the restoring force of a spring arranged between the piston and the countershaft. The control pressure acting in the pressure chamber is controlled by means of an inlet valve connected on the inlet side to a pressure line and an outlet valve connected on the outlet side to an unpressurized line, which on the outlet and inlet sides are respectively connected to the pressure chamber of the brake cylinder by way of a short duct in each case. In this case the two valves are in the form of 2/2-way magnetic switching valves, which are relatively inexpensive and which enable simple control sequences. Since in the deactivated condition the transmission brake should reliably remain open without energy consumption, in the non-actuated, i.e. de-energized condition the inlet valve is closed whereas in the non-actuated condition the outlet valve is open.
During the synchronization process of an upshift the two valves are generally controlled in such manner that when the loaded gear has been disengaged, at approximately the same time the inlet valve is opened and the outlet valve is closed. Thereby the pressure medium flows out of the pressure line into the pressure chamber of the transmission brake which is closed on the outlet side and the piston presses the inner and outer disks against one another, so that a braking torque is produced which brakes the input shaft. When the braking torque required for synchronizing the target gear has built up, the inlet valve is closed. This traps the pressure medium inside the pressure chamber of the transmission brake, whereby the braking torque of the transmission brake is kept constant. To reach the synchronous speed and avoid over-braking the input shaft, the outlet valve is opened at just the right time before the synchronous speed has been reached so that the pressure medium can flow out of the pressure chamber of the transmission brake into the unpressurized line, which causes the braking torque to fall, i.e. the transmission brake is deactivated.
Previous known methods for controlling a transmission of this type are limited, during a braking of the input shaft necessitated by an upshift, with a braking gradient applied by the transmission brake, to determining the optimum time for opening the outlet valve, i.e. for deactivating the transmission brake.
DE 102 24 064 B4 describes a corresponding method for controlling a transmission brake, in which when the transmission brake has been activated, the input rotational speed is extrapolated by means of the input speed gradient and the deactivation time of the transmission brake is determined in such manner that when the target gear is engaged, the input speed corresponds within a specified tolerance to the synchronous speed determined by the output speed. For the determination of the deactivation time a deactivation lag time of the transmission brake and an output speed gradient are taken into account, which are attributable to a resultant resistance torque acting on the output shaft and which give rise to a corresponding change of the synchronous speed. However, in this known method the reduction of the braking torque during the deactivation of the transmission brake is perceived as an unsteady or abrupt process that does not exactly match reality and leads to a certain imprecision of the method.
In contrast, in the method known from DE 10 2010 002 764 A1 for controlling a transmission brake it is provided that for the determination of the deactivation time of the transmission brake, in addition to a deactivation lag time of the transmission brake and an output rotational speed gradient, i.e. a change of the synchronous speed, a steady reduction of the braking torque during the deactivation process of the transmission brake is also taken into account. For this the reduction of the input speed gradient brought about by the braking torque of the transmission brake is described by a quadratic time function whose quadratic portion is weighted by a transmission-specific and brake-specific deactivation factor FAbs of the transmission brake. This improved method enables a substantially more accurate determination of the deactivation time of the transmission brake.
Basically, a synchronization process carried out by the transmission brake during an upshift should take place as quickly as possible. However, to be able to reliably determine the optimum time for opening the outlet valve, i.e. for deactivating the transmission brake, the input speed gradient has to be determined relatively precisely. But for an accurate determination of the input speed gradient from the speed signal of a rotational speed sensor arranged on the input shaft, a minimum steady application time of the transmission brake is necessary during which the braking torque of the transmission brake and hence the input speed gradient are substantially constant, since when the engine clutch is opened and the loaded gear is disengaged the input shaft usually undergoes rotation fluctuations and the rotational speed signal concerned can be ‘noisy’.
During the synchronization of the target gear by means of the transmission brake it should also be taken into account that the supply pressure in the pressure line of the transmission brake can fluctuate, so that the maximum braking torque of the transmission brake that can be set is limited. For example, this is the case if the pressure line is connected not to a system pressure line with a largely constant, high system pressure, but to a shifting pressure line of the transmission in which, by virtue of an associated pressure regulating valve, shift-dependent shifting pressures of varying size are produced. However, to produce a particular braking torque by means of the transmission brake, if the supply pressure is lower a longer opening duration of the inlet valve is needed that with a higher supply pressure. In addition the production of a particular braking torque is made more difficult because the transmission brake is not usually provided with a pressure sensor by means of which the brake pressure present in the pressure chamber of the transmission brake could be determined.